A network switch is a computer networking device that connects network segments.
The term commonly refers to a Network bridge that processes and routes data at the Data link layer (layer 2) of the OSI model. Switches that additionally process data at the Network layer (layer 3 and above) are often referred to as Layer 3 switches or Multilayer switches.
The term network switch does not generally encompass unintelligent or passive network devices such as hubs and repeaters.
The first Ethernet switch was introduced by Kalpana in 1990.

Function
The network switch, packet switch (or just switch) plays an integral part in most Ethernet local area networks or LANs. Mid-to-large sized LANs contain a number of linked managed switches. Small office, home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as gateway access to small office/home office broadband services such as DSL router or cable Wi-Fi router. In most of these cases, the end user device contains a router and components that interface to the particular physical broadband technology, as in the Linksys 8-port and 48-port devices. User devices may also include a telephone interface to VoIP.
In the context of a standard 10/100 Ethernet switch, a switch operates at the data-link layer of the OSI model to create a different collision domain per switch port. If you have 4 computers A/B/C/D on 4 switch ports, then A and B can transfer data between them as well as C and D at the same time, and they will never interfere with each others' conversations. In the case of a "hub" then they would all have to share the bandwidth, run in Half duplex and there would be collisions and retransmissions. Using a switch is called micro-segmentation. It allows you to have dedicated bandwidth on point to point connections with every computer and to therefore run in Full duplex with no collisions.

Role of switches in networks
Network switch is a marketing term rather than a technical one.[citation needed] Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch, although use of the term is diminishing.[citation needed]
In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the layers mentioned. While Layer 2 functionality is adequate for speed-shifting within one technology, interconnecting technologies such as Ethernet and token ring are easier at Layer 3.
Interconnection of different Layer 3 networks is done by routers. If there are any features that characterize "Layer-3 switches" as opposed to general-purpose routers, it tends to be that they are optimized, in larger switches, for high-density Ethernet connectivity.
In some service provider and other environments where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall,network intrusion detection, and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.
In other cases, the switch is used to create a mirror image of data that can go to an external device. Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems and packet sniffers.

Network security consists of the provisions made in an underlying computer network infrastructure, policies adopted by the network administrator to protect the network and the network-accessible resources from unauthorized access, and consistent and continuous monitoring and measurement of its effectiveness (or lack) combined together.

Comparison with information security
The terms network security and information security are often used interchangeably, however network security is generally taken as providing protection at the boundaries of an organization, keeping the intruders (e.g. black hat hackers, script kiddies, etc.) out. Network security systems today are mostly effective, so the focus has shifted to protecting resources from attack or simple mistakes by people inside the organization, e.g. with Data Loss Prevention (DLP). One response to this insider threat in network security is to compartmentalize large networks, so that an employee would have to cross an internal boundary and be authenticated when they try to access privileged information. Information security is explicitly concerned with all aspects of protecting information resources, including network security and DLP.

Network security concepts
Network security starts from authenticating any user, commonly (one factor authentication) with a username and a password (something you know). With two factor authentication something you have is also used (e.g. a security token or 'dongle', an ATM card, or your mobile phone), or with three factor authentication something you are is also used (e.g. a fingerprint or retinal scan). Once authenticated, a stateful firewall enforces access policies such as what services are allowed to be accessed by the network users.[1] Though effective to prevent unauthorized access, this component fails to check potentially harmful content such as computer worms being transmitted over the network. An intrusion prevention system (IPS)[2] helps detect and inhibit the action of such malware. An anomaly-based intrusion detection system also monitors network traffic for suspicious content, unexpected traffic and other anomalies to protect the network e.g. from denial of service attacks or an employee accessing files at strange times. Communication between two hosts using the network could be encrypted to maintain privacy. Individual events occurring on the network could be tracked for audit purposes and for a later high level analysis.
Honeypots, essentially decoy network-accessible resources, could be deployed in a network as surveillance and early-warning tools. Techniques used by the attackers that attempt to compromise these decoy resources are studied during and after an attack to keep an eye on new exploitation techniques. Such analysis could be used to further tighten security of the actual network being protected by the honeypot.
A useful summary of standard concepts and methods in network security is given by in the form of an extensible ontology of network security attacks.

Security management
Security Management for networks is different for all kinds of situations. A small home or an office would only require basic security while large businesses will require high maintenance and advanced software and hardware to prevent malicious attacks from hacking and spamming.

Small homes
A basic firewall like COMODO Internet Security or a unified threat management system.
For Windows users, basic Antivirus software like AVG Antivirus, ESET NOD32 Antivirus,Kaspersky, McAfee, or Norton AntiVirus. An anti-spyware program such as Windows Defender or Spybot would also be a good idea. There are many other types of antivirus or antispyware programs out there to be considered.
When using a wireless connection, use a robust password. Also try and use the strongest security supported by your wireless devices, such as WPA or WPA2.
If using Wireless: Change the Default SSID network name, also Disable SSID Broadcast; as this function is unnecessary for home use.
Enable MAC Address filtering to keep track of all home network MAC devices connecting to your router.
Assign STATIC IP addresses to network devices.
Disable ICMP ping on router.
Review Router or Firewall logs to help identify abnormal network connections or traffic to the Internet.
Use passwords for all accounts.
Have multiple accounts per family member, using non-administrative accounts for day-to-day activities. Disable the guest account (Control Panel> Administrative Tools> Computer Management> Users).
Raise awareness about information security to children.

Medium businesses
A fairly strong firewall or Unified Threat Management System
Strong Antivirus software and Internet Security Software.
For authentication, use strong passwords and change it on a bi-weekly/monthly basis.
When using a wireless connection, use a robust password.
Raise awareness about physical security to employees.
Use an optional network analyzer or network monitor.
An enlightened administrator or manager.

Large businesses
A strong firewall and proxy to keep unwanted people out.
A strong Antivirus software package and Internet Security Software package.
For authentication, use strong passwords and change it on a weekly/bi-weekly basis.
When using a wireless connection, use a robust password.
Exercise physical security precautions to employees.
Prepare a network analyzer or network monitor and use it when needed.
Implement physical security management like closed circuit television for entry areas and restricted zones.
Security fencing to mark the company's perimeter.
Fire extinguishers for fire-sensitive areas like server rooms and security rooms.
Security guards can help to maximize security.

School
An adjustable firewall and proxy to allow authorized users access from the outside and inside.
Strong Antivirus software and Internet Security Software packages.
Wireless connections that lead to firewalls.
Children's Internet Protection Act compliance.
Supervision of network to guarantee updates and changes based on popular site usage.
Constant supervision by teachers, librarians, and administrators to guarantee protection against attacks by both internet and sneakernet sources.

Large Government
A strong firewall and proxy to keep unwanted people out.
Strong Antivirus software and Internet Security Software suites.
Strong encryption.
Whitelist authorized wireless connection, block all else.
All network hardware is in secure zones.
All host should be on a private network that is invisible from the outside.
Put all servers in a DMZ, or a firewall from the outside and from the inside.
Security fencing to mark perimeter and set wireless range to this.